Field of the Invention (Technical Field)
The presently claimed invention relates to solar energy production and more particularly to a method and apparatus for constructing mechanically linked, single axis solar tracking systems of various tracking geometries to follow the diurnal motion of the sun.
Background Art
Solar tracking systems utilized in renewable energy production are devices that track the motion of the sun relative to the earth to maximize the production of solar energy. Solar trackers move to keep solar modules perpendicular to the sun in either one or two axes. The presently claimed invention applies to photovoltaic modules (PV) for generating electrical power, but may be applied to any solar energy collection device, such as solar thermal or materials exposure testing devices. Solar trackers have been successfully deployed in the industry; however, the prior art designs have not adequately addressed the initial installation costs, flexibility in adaptation to site conditions, and reliability over the relatively long lifetime (20+ years) of the system. In choosing a solar tracker system, one must consider all of the following variables:                PV module cost,        land cost, site geometry and availability,        installation labor cost,        material cost,        meteorological data,        operation and maintenance costs,        overall efficiency increase that the tracker provides.        
The state of the art approaches have not fully optimized the combination of all the relevant cost issues. The energy gain provided by trackers is dependent upon the tracking geometry of the system and the location of the installation. A dual axis (D/A) tracker keeps the collector perpendicular to the sun on both axis', and provides the greatest gain in energy production at any location. Single axis (S/A) trackers are fixed in one axis and typically track the daily motion of the sun in the other axis. Single axis tracker geometries include tilted elevation, azimuth, and horizontal. Tilted elevation S/A trackers are tilted as a function of the location's latitude and track the sun's daily motion about that tilted axis. Azimuth S/A trackers are tilted at an optimum angle and follow the daily motion of the sun by rotating about the vertical axis. Horizontal S/A trackers are configured parallel to the ground and rotate about a North/South horizontal axis to track the sun's daily motion. The energy gained varies for each type of tracking geometry and is dependent upon the latitude of the installation and the weather conditions at the installation location. Solar tracking systems for PV modules are commercially available in single axis tilt and roll, single axis horizontal, single axis fixed tilt azimuth, and dual axis geometries.
All trackers must be built strong enough to resist the wind forces in any tracking position or be “stowed” to reduce the effect of extreme wind forces. Modules also require periodic cleaning, which in many locations is primarily accomplished by rain “washing” the modules. Snow can impact tracker operations, due to the occurrence of ice or the weight of snow on modules, or snowdrifts that interfere with tracker movement and the collection of solar energy. In addition, construction materials, electronics, drive components, and motors must be able to operate within temperature and climate constraints.
In many applications, the horizontal single axis tracker is the most cost effective tracker geometry. A horizontal S/A tracker structure may be supported at many points along the rotating axis and, therefore, requires less complexity and less material for construction than other tracking geometries. The key to successful design of a tracking apparatus for PV modules is to provide the maximum overall economic benefit, such as the initial apparatus cost, the installation cost, the land utilization, the cost and efficiency of the solar modules, and the operation and maintenance costs as well as the efficiency gain provided by the tracking geometry. As the cost of steel and other fabrication material rises, the horizontal tracking geometry is increasingly desirable. It minimizes the structural material requirements by keeping the modules at a relatively low profile to the foundation, and at a minimum overhung moment load relative to the rotating axis without requiring special connections to rotate the system about its center of gravity.
The prior art horizontal axis trackers typically have connected each row of modules together with a linear motion linkage in an effort to minimize the number of drive motors required. Prior art mechanically linked horizontal and tilted single axis tracking systems require substantial mechanical linkages structurally capable of resisting high force loading due to overhung solar module weight and large forces induced by the wind.
The shortcoming of this prior art system is that all of the wind forces are concentrated to a single point, through the mechanical linkage. The embodiment of the presently claimed invention specifically eliminates the need for a robust mechanical linkage capable of resisting high-load forces induced by the wind. The design of the current embodiments eliminates the transmittance of these wind forces to the linkage, and counteracts the external wind forces locally, within each tracker row or array such that the wind force is not transmitted to the linkage. The prior art also requires a separate, large foundation, or foundations, to anchor a single drive mechanism that rotates many rows of modules with a linear motion motor. One such device is a horizontal, single axis tracking system described in U.S. Pat. No. 6,058,930, to Shingleton. In this system, the horizontal rows of modules are linked together with a linear motion linkage and operated by a single linear actuator attached to a separate, large foundation. In addition to the prior art horizontal axis, mechanically linked trackers require generally fiat or graded terrain for proper operation. Many columns must be installed at height elevations and locations requiring high tolerance within 100+ columns, across two dimensions in a large area, for mechanical linkages between rows to line up for operation. This often requires extensive and costly site preparation. Some prior art linked horizontal trackers have embodiments that allow for installation on undulating terrain, but require expensive joints that must be fabricated onsite that also must resist the large forces induced by the wind. These high force loaded pivoting joints are generally complicated and expensive to construct. Another disadvantage of the prior art is that they are designed as large rectangles with a linkage running down the center of the array field. If the installation field is not suitable in the shape of a rectangle, these systems are often employed in less than optimum configurations where fewer modules are controlled by the linkage. This is another cost increase factor for the prior art in many installations. The linear motion linkage of the prior art represents an excess of material and a labor-intensive installation cost component. The linkage must be robust in order to directly resist the force of an entire field of many rows of trackers to one large linear drive that must be affixed to a large separate linear actuator drive foundation. The separate, large foundation is necessary to anchor the drive mechanism and must resist very high forces induced by the wind to the entire tracker field. In addition, the flexibility in site layout is impacted by the linear motion linkage since the drive connection must run generally centered in the rows and be installed in a straight perpendicular line. The mechanical linkage of the prior art must be fixed at a right angle to the torsion tube and cannot deviate from perpendicular, therefore, not allowing the system to conform to irregular installation site boundaries.
Tracking geometries other than the horizontal single axis require more land area for installation. In a field of trackers, all the tracker geometries except for the horizontal axis tracker must be spaced in two dimensions, East/West and North/South, so as not to shade each other. The horizontal axis tracker need only be spaced apart in the East/West dimension to alleviate shading and, therefore, requires much less land to implement. Land contour and shape also critically control the cost of the installation of most horizontal single axis tracker systems.
Another type of horizontal axis tracker is not linked together and typically includes multiple PV modules mounted astride a torque tube. These are designed as independently motor driven rows. These horizontal trackers are driven individually by a motor/gear drive system and the PV array is rotated about the center of gravity of the PV module tracking system. Rotating the array about the center of gravity eliminates the moment loads applied to the gear drive by the overhung weight of the solar modules. In order to rotate the array about the center of gravity, this type of horizontal tracker design requires more structural material and more costly torque tube connections and bearings than the present horizontal axis tracker embodiments. Other disadvantages of these tracker designs include a higher projected wind area that requires more structural material and large foundations to resist greater moment loads and larger capacity drives to overcome moment loading from the solar modules that are mounted at a larger distance from the torque tube due to the taller profile of the array. They also have more complex bearing and support points that rotate the PV modules about the center of gravity of the tracker, and use a motor per single tracker row, which equates to increased cost, maintenance, and decreased reliability.
A third tracker geometry is a tilted, single axis tracker. Often termed a tilt and roll tracker, it is tilted in elevation and then rotates about that tilted axis. This type of tracker typically offers increased gain over a horizontal tracking system, but at an added cost that should be critically analyzed prior to deployment. These costs include the requirement for more land due to the spacing necessary for shading in both the N/S and E/W dimensions and a more complex structure requiring more structural material because of increased projected height from foundation. These systems are also not capable of automatic stow during high winds since the elevation angle is fixed and therefore, must be structurally capable of withstanding all wind forces. Another tilted single axis geometry is a fixed tilt azimuth tracker. A fixed tilt azimuth tracker is tilted in elevation and then rotates about a vertical axis. This design, although typically more structurally stable than a tilt and roll tracker, suffers from the same cost drawbacks as the tilt and roll design; although, the performance gain may make the tilted single axis geometry economic for some installations.
The last tracking geometry is a dual-axis (D/A) tracker. D/A trackers provide the greatest performance gain over all the aforementioned tracking geometries since they keep the solar modules perpendicular to the sun in both axes. There are; however, several practical disadvantages of these systems: more land is required due to spacing necessary for shading in two dimensions; a more complex structure is necessary that requires more structural material as a result of increased projected height from the earth and foundation; and a second drive axis for elevation is necessary, which increases complexity, expense, and maintenance issues. Also, D/A systems typically use two drive motors per a relatively small surface area of solar modules that results in increases in both initial cost and subsequent maintenance costs. Some types of solar collectors, concentrating collectors for example, require D/A tracking to operate.
As previously indicated, an ideal solar tracking system will operate in all types of conditions. This includes situations whereby a tracker's movement is impeded by obstructions or the like. If there are no safeguards in place, permanent damage can result in the tracker system when an obstruction condition exists. In addition, human intervention may be necessary to cure the condition. Sometimes, timely human intervention is impossible if the trackers are in remote locations and secondly, sending out a technician for every obstruction condition can be very costly.
Another need in a solar tracker system is the ability to have flexibility in the design of the systems to support different lengths of driveshafts for differing terrain conditions and systems. Presently, the fabrication of specific length of driveshafts requires field welding and painting. A similar problem exists for torsion tubes. A design for providing a simple method to join torsion tube segments together in the field is necessary.